Semiconductor technologies often form extremely clean microstructure surfaces on microelectromechanical devices (often referred to as “MEMS devices”). Undesirably, extremely clean microstructure surfaces often stick together if they come into contact.
When the surfaces remain stuck together, the device often is inoperable. This concept of surface sticking is known in the art as “stiction.”
A number of different factors can contribute to stiction. For example, among other things, stiction can be caused by capillary, electrostatic, and van der Waals forces. In some cases, stiction can be caused by “chemical” forces, such as hydrogen bonding and solid bridging. Accordingly, prior to distribution of a MEMS device, MEMS device manufacturers often perform a variety of tests to determine the potential for stiction problems.
In order to combat stiction, the wafer surface may be coated with a thin anti-stiction coating having a low surface energy. Examples of common anti-stiction coatings include hydrocarbon and fluorocarbon based self assembled organosilanes and siloxanes applied either in solvent or via chemical vapor deposition. The anti-stiction coating typically covers the entire wafer surface including such things as bond lines used for wafer-to-wafer bonding (e.g., wafer capping or stacking) and bond pads for electrical and other connections.
MEMS devices are often sealed in hermetic enclosures by either putting each individual dies in hermetic packages, or encapsulating sensors in wafer level. Wafer level encapsulation can reduce package size and cost, and it has often been achieved by using seal glass (or glass frit) as an intermediary bonding layer. Although the seal glass bonding has been incorporated in many MEMS products, including accelerometers, it imposes several constraints including larger die size on the order of 100 micron, contamination of die enclosure during screen printing, and contamination from lead used to reduce the melting temperature of glass.
Other types of bonding, such as metal-to-metal, Si-to-Si, and Si-to-metal bonding, may be used as an alternative to seal glass bonding. Some examples include copper-to-copper, aluminum-to-aluminum, and aluminum/germanium-to-aluminum bonding using thermal compression bonding schemes. Such alternative bonding techniques typically allow for reduced bond line thickness, which can allow for more dies per wafer and hence lower cost per die.
After bonding the cap and device wafers together, the capped wafers are “singulated” or diced into individual capped devices. During singulation, the un-bonded areas between the caps (often referred to as “grid”) becomes loose and after dicing, the grid is removed, e.g., by taping the cap wafer from the top and pulling off the grid. Quantitative bond yield and strength may then be evaluated, for example, by inspecting the grid to determine the percentage of bonded and un-bonded grid using a XYZTEC™ shear measurement tool.
Wafer-to-wafer bonding is typically done with the anti-stiction coating still intact and covering the bond lines, for example, using thermocompression bonding. However, the existence of the anti-stiction coating on the bond lines can negatively impact adhesion and can reduce the bond shear strength, which can result in an incomplete or insufficient wafer-to-wafer bond. These things can lower yield (i.e., the number of acceptable devices realized per wafer) and also can lead to device failures.
Plasma etching has been used to remove anti-stiction coating material from bond line surfaces, but plasma etching is generally non-selective and can result in removal of anti-stiction coating materials from movable MEMS structures that require stiction prevention.